1. Technical Field
The present invention relates to an eccentric valve, and particularly to an eccentric valve having a tri-offset structure.
2. Description of the Prior Art
A butterfly valve, which is a commonly-used fluid valve, includes an axially-penetrating shaft hole for the insertion of a drive shaft whereby a valve unit of the butterfly valve is driven and rotated, so the valve unit can be opened or closed, wherein the valve unit is designed to have a tri-offset structure. A conventional eccentric valve includes a valve body having a valve chamber; a valve stem penetrating the valve body; a valve seat received in the valve chamber; a seal which is an O-ring, and is asymmetric on both ends of a radial section thereof; a valve unit provided with a shaft hole in an axial direction thereof, the shaft hole being configured to allow the valve stem to penetrate therethrough for controlling the opening and closing of the valve unit. The seal is disposed on the valve unit and configured to be tightly fit to the valve seat when the valve unit is closed.
The eccentric valve is provided with a tri-offset structure. The seal diverges from the paired shaft holes of the valve unit (i.e., diverges from the valve stem), and the radial centerline of the seal is away from the center of the valve stem by a vertical distance, forming a first offset. Next, a second axial centerlines of the valve stem is away from a first axial centerline by a distance, forming a second offset. Next, an inner surface of the valve seat extends to form a tapered shape, the vertical distance between the peak of the tapered shape and the first axial centerline being not greater than the radius of the valve unit, thus forming a third offset. The tri-offset structure has been clearly described in FIG. 4 and description of the issued U.S. Pat. No. 5,975,494. Conventionally, a side of the tapered shape is parallel to the axial direction of piping, the seal therefore needs to be machined according to various angles of the piping to form an angle providing tight fitting.
Conventionally, the valve seat is tailored to the design of the tri-offset structure. The slope of the inner side of the valve seat needs to match the slope of the aforementioned offset tapered shape. In other words, the upper part of the inner side of the valve seat is oblique, while the lower part of the inner side of the valve seat is horizontal. These two slopes match the slopes of the tapered shape, and the slopes of the inner side of the entire valve seat continuously vary from the top down.
With the aforementioned tapered shape design, an annular seal needs to be formed as a radially asymmetric circle with respect to its axis, in order to enable the seal to tightly seal the valve body. As such, when the circularly asymmetric seal is mounted on the outer side of the valve unit against the valve set, the seal can firmly contact the inner side of the valve seat at various positions on the edge of the seal, forming a tight sealing. Because the seal is a radially asymmetric circle with respect to its axis, it is difficult to be manufactured. Also, the mounting of the seal is directional. When being mounted, such seal needs to be inserted into the valve body. However, as the seal slides, the tight sealing will be damaged, causing leakage.